; blinkdi.asm -- resident blink hooked on the display interrupt, via the
; firmware's own RAM trampoline at bit 00000DB0h.
;
; HOW IT HOOKS
;   The CPU vector table lives in ROM, but RLONG(0xFFFFFEA0) = 00000DB0h and
;   that RAM word is a 6-byte `JA <handler>` stub.  `install` rewrites the
;   stub's 32-bit target to `entry` (the opcode C080h is unchanged).  The
;   original target 3F7146D0h is the OS display handler.
;
; WHY THE RETURN-PC TRAMPOLINE
;   A one-shot VRAM draw is repainted before it reaches the screen.  Drawing
;   inside the handler *before* the OS handler runs is also wiped by the
;   handler's own full-screen work.  So:
;     entry  (runs first) saves the interrupt's return PC, replaces it with
;            `ret_hook`, then tail-jumps to the stock OS handler;
;     ret_hook (runs after the OS handler's `reti`) draws the box and jumps
;            back to the saved PC.  The draw is therefore after the repaint.
;
; The box is at framebuffer (256,400), 64x64, on the Terminal Manager's white
; body.  State toggles every 0600h invocations (~48 frames at the measured
; ~32 display interrupts/frame): black <-> white.
;
; Linked at bit 01840000h by dwt/visual/build.sh.

	.text
_start:					; install, called by rdebug `jump`
	pushst
	dint
	setf	32, 0, 0
	movi	entry, a0, L
	move	a0, @00000DC0h, 0	; stub target (32 bits, two words)
	setf	16, 0, 0
	movi	0C080h, a0, L		; JA opcode (unchanged value)
	move	a0, @00000DB0h, 0
	sub	a0, a0
	move	a0, @0186F000h, 0	; invocation counter = 0
	move	a0, @0186F002h, 0	; state = 0 (black)
	popst
	rets

; ---- entry hook: save return PC, install ret_hook, run stock handler ------
entry:
	move	a0, @0186F010h, 1	; save interrupted a0
	setf	32, 0, 1
	move	*sp(020h), a0, 1	; saved PC (SP -> ST, +20h -> PC)
	move	a0, @0186F004h, 1	; orig_pc
	movi	ret_hook, a0, L
	move	a0, *sp(020h), 1	; return via ret_hook instead
	move	@0186F010h, a0, 1	; restore a0
	ja	3F7146D0h		; stock OS display handler

; ---- ret hook: after the OS handler, draw, then go back -----------------
ret_hook:
	move	a0, @0186F010h, 1
	getst	a0
	move	a0, @0186F014h, 1	; save interrupted ST
	dint
	move	a1, @0186F020h, 1
	move	a2, @0186F024h, 1
	move	a3, @0186F028h, 1
	move	a4, @0186F02Ch, 1
	move	a5, @0186F030h, 1
	move	b0, @0186F034h, 1

	setf	16, 0, 0
	setf	32, 0, 1
	move	@0186F000h, a0, 0	; counter
	inc	a0
	move	a0, @0186F000h, 0
	cmpi	0600h, a0
	jrne	draw			; only flip state on wrap
	sub	a0, a0
	move	a0, @0186F000h, 0
	move	@0186F002h, a1, 0	; current state
	cmpi	0, a1
	jrne	was_white
	movk	1, a1			; 0 -> 1 (white)
	jruc	set_state
was_white:
	sub	a1, a1			; 1 -> 0 (black)
set_state:
	move	a1, @0186F002h, 0

draw:
	move	@0186F002h, a1, 0
	cmpi	0, a1
	jrne	pat_white
	sub	a2, a2			; black
	jruc	pat_ready
pat_white:
	movi	0FFFFh, a2, L		; white
pat_ready:
	movi	04064100h, a3, L	; 04000000h + 400*1024 + 256
	movi	64, a4, L		; 64 rows
	movi	4, a5, L		; 4 words = 64 px
drow:
	move	a5, a1
dword:
	move	a2, *a3+, 0
	dsjs	a1, dword
	addi	03C0h, a3		; 1024 - 64 bits
	dsjs	a4, drow

	move	@0186F030h, a5, 1
	move	@0186F02Ch, a4, 1
	move	@0186F028h, a3, 1
	move	@0186F024h, a2, 1
	move	@0186F020h, a1, 1
	; leave with orig_pc on the stack and every touched register restored
	move	@0186F004h, a0, 1	; orig_pc
	move	a0, -*sp, 1		; push return PC for `rets`
	move	@0186F010h, a0, 1	; restore interrupted a0
	move	@0186F014h, b0, 1	; interrupted ST
	putst	b0			; restore IE/fields
	move	@0186F034h, b0, 1	; restore interrupted b0
	rets				; pop orig_pc and resume
	.end